Somatic Polyploid Petals: Regeneration Offers New Roads for Breeding Belgian Pot Azaleas

Bud sporting, the consequence of sudden variations in gene expression of somatic cells, leads to the occurrence of phenotypically altered shoots in many vegetatively propagated plant species. In ornamentals, such as azalea ( Rhododendron simsii ), flower colour bud sports are appreciated as a valuable additional source of variation, and it is evident that both genetic and epigenetic mechanisms are involved in the creation of the whole colour range. Flowers exhibiting two-coloured petals, i.e. the margin is coloured differently than the central zone, characterize one of the sport types, namely picotees. This particular sporting type contributes to the enlargement of the available variation not only because of its coloration pattern but interestingly displays somatic polyploidy in its petals. The differently coloured petal margin is tetraploid while the rest of the corolla, as well as the entire plant are diploid. Aiming at introducing polyploidy in the completely diploid breeding collection, in vitro regeneration of tetraploid petal marginal tissue was performed. This led to the production of the first induced tetraploid Belgian azalea. The markedly increased robustness of the flowers as well as their polyploid status is exceptionally interesting for breeding programs.